This invention relates to a semiconductor device comprising an isolation layer for electrically isolating constituent elements by means of a PN junction and/or a high resistance layer.
A passive element such as a high resistance layer or isolation layer for electrically isolating constituent elements by means of a PN junction is generally formed by any of the undermentioned processes.
(1) A first process runs as follows: PA0 (2) A second process is carried out by the following steps. PA0 (3) A third process comprises the steps of selectively forming the photoresist on the thermally oxidized layer, selectively implant boron in the aforesaid N type epitaxially grown layer by ion implantation through the thermally oxidized layer, followed by heat treatment of 1100.degree. C. to 1200.degree. C.
An N type layer is formed by epitaxial growth, for example, on a P type substrate. A thermally oxidized layer is mounted on the surface of the epitaxially grown N type layer. The oxidized layer is selectively etched with a photoresist used as an etching mask to form an opening. A boron silicate glass (BSG) layer is spread over the surface of the oxidized layer including the opening by the chemical vapor deposition method. Boron is thermally diffused as an impurity from the BSG layer in the aforesaid epitaxially grown N type layer through the opening.
After the above-mentioned opening is formed, boron is diffused similarly as an impurity in the aforesaid epitaxially grown N type layer from, for example, a solid impurity source of boron nitride at a high temperature of 1100.degree. to 1200.degree. C., using a carrier gas.
Where a high resistance layer is formed by the first or second process with the BSG layer or boron nitride layer used as a source of an impurity, then the sheet resistances of the plural high resistance layers formed in a given substrate including the epitaxially grown N type layer vary at as high a rate as .+-.15%. Further, the sheet resistances of the plural high resistance layers vary at the same high rate between the respective substrates. Therefore, it is very difficult to control the sheet resistances of the high resistance layers. With the third process applying the ion implantation of boron, the sheet resistances of the high resistance layers vary to a far smaller extent than in the first and second processes namely, by 1/2 to 1/3 of the above-mentioned extent of .+-.15%. Where, however, boron or gallium belonging to the III group of the periodic table or phosphorus or arsenic belonging to the V group of the periodic table is implanted by ion implantation, then the sheet resistance of a high resistance layer thus formed is limited within the range of 2000 .OMEGA./.quadrature.. It is difficult to cause a high resistance layer to maintain a higher resistance than this level, because considerable difficulties are encountered in controlling the degree of resistance based on the content of an impurity in the epitaxially grown high resistance layer.